The present invention relates to an article based on a metal alloy of nickel, chromium and metalloid elements including microcrystalline precipitates, and the relative preparation method.
It is known in the art that metal superalloys based on nickel allow performances characterized by an extremely high mechanical resistance at high temperatures; in fact, alloys based on nickel-chromium, for example, are used for the construction of blades in gas turbine rotors. The use of this group of alloys in resistors is also known, owing to the high resistivity with respect to conventional metal conductors, the low variation resistivity coefficient with temperature and the above-mentioned mechanical resistance at high temperature.
Furthermore, some particular compositions of these alloys (with respect to the Ni/Cr ratio) are used in the electro-technical field owing to their reduced or absence of magnetic susceptibility.
It is also known that the melting point of these alloys can be lowered by several hundreds of degrees, by adding metalloid elements such as phosphorous, boron or silicon. The alloys thus obtained are particularly suitable for the brazing of steel or other superalloys based on nickel, in particular in the form of fine sheets having a thickness of less than 50 xcexcm, which are prepared by means of the rapid solidification technology, for example on a cooled rotating wheel (melt spinning or planar flow casting) (U.S. Pat. No. 4,148,973).
The presence of metalloids in the alloy and the high solidification rate cause the amorphization of the tapes or sheet thus produced, which have particularly high mechanical properties only below the crystallization temperature i.e. typically below 300-400xc2x0 C. Over these temperatures the end-products thus obtained become fragile and it is therefore not possible to use them in applications which require a structural resistance at high temperatures.
This problem is known in particular and the state of the art (Tung S. K. et al., Scripta Materialia, 345, 1996) indicates that this fragility of superalloys containing metalloids, is due to the formation of intermetallic compounds which are formed in the above alloys. Efforts have consequently been made to solve this problem by introducing the minimum possible quantity of said metalloids, i.e. the absolute quantity necessary for amorphization. From what is specified above, it is evident that the use of superalloys in films or fine sheets, having a thickness of less than 50 xcexcm, is extremely limited owing to the problems described above.
The present invention therefore proposes to overcome the drawbacks present in the known art.
It has been surprisingly found, in fact, that by using alloys based on nickel-chromium, with a low or no magnetic susceptibility (i.e. with a higher chromium content), containing metalloid elements, such as boron and silicon, in a higher quantity with respect to what is described in the known art, and by subjecting the above alloys to a particular thermal treatment after rapid solidification it is possible to produce tapes or filiform end-articles having interesting mechanical properties and resistance to oxidation, without the fragility typical of materials having a similar composition crystallized at temperatures only slightly higher than the glass transition temperature.
An object of the present invention therefore relates to an article based on a nickel-chromium-silicon metal alloy, including microcrystalline borides, obtained by the rapid solidification and subsequent thermal treatment , at a temperature ranging from 700xc2x0 C. to 950xc2x0 C. for a time which varies from 5 minutes, in the upper temperature limit, to 100 hours in the lower temperature limit, of a nickel-chromium-boron-silicon metal alloy comprising from 39.0 to 69.4 atom % of nickel, from 11.8 to 33.9 atom % of chromium, from 7.6 to 27.4 atom % of boron and from 7.6 to 17.5 atom % of silicon.
The thermal treatment is carried out within the range of 700 to 950xc2x0 C., bearing in mind that below the lower temperature limit, the duration times are very long, over hundreds of hours and therefore of limited industrial interest, whereas over the upper temperature limit, coalescence phenomena of the precipitates arise, which reduce the mechanical properties. In the intermediate temperature range the treatment has an adequate duration, in the order of hours, with longer times at low temperatures and shorter times at high temperatures.
A further object of the present invention relates to a tape-form or filiform article having a thickness of less than 50 xcexcm.
An additional object of the present invention relates to the use of articles in the form of non-magnetic tapes as substrates for the growth of superconductor oxides.
An object of the present invention also relates to the use of articles in tape, sheet or fiber form as a reinforcing element in composite materials with an organic, metallic or pyroceram matrix.
In particular, the tape-form or filiform article according to the present invention is characterized in that it has a thickness ranging from 5 to 40 xcexcm.
The thermal treatment according to the present invention is preferably carried out in an inert gas or under vacuum, at a temperature preferably ranging from 750xc2x0 C. to 880xc2x0 C. and for a time which varies from about 30 minutes (at the upper limit of the temperature range) to about 15 hours (at the lower limit of the temperature range).
The very particular properties of the article according to the present invention are obtained as a result of the rapid solidification and subsequent thermal treatment to which the article is subjected.
This treatment, in fact, allows a structure to be obtained, which is characterized by the presence of microcrystals of nickel and chromium borides, precipitated in the metallic matrix of the nickel, chromium and silicon constituents. If the thermal treatment is carried out in air, the article, on oxidizing, has a nickel, chromium and silicon elemental composition of the matrix, which varies in the different parts of the article itself.
In particular, the use of articles in non-magnetic tape form as substrates for the growth of superconductor oxides, is especially interesting.
The possibility of obtaining superconductor tapes by deposition, with physical or chemical methods, of superconductor oxides on metallic substrates, is known, but the use of this metallic substrate has various problems.
The metallic substrate, in fact, must not only be non-magnetic (a necessary quality for guaranteeing low stream and/or varying magnetic field losses, in regime), but must also be extremely fine (a few tens of micrometers at the most), in order to obtain a high volumetric ratio between the fine superconductor film (in the order of micrometers) and the metallic substrate. In addition, the metallic substrate should not be reactive with the superconductor oxide at the high temperatures typical of its crystalline growth, i.e. at temperatures ranging from 800xc2x0 C. to 900xc2x0 C. In order to satisfy all the above conditions, the metallic substrates are generally produced by means of laborious rolling techniques and sophisticated deposition technologies of protective films, as regards both the oxidation of the substrate, and also with respect to the migration of metallic elements from the substrate to the superconductor oxide.
This use of the article according to the present invention is particularly advantageous in that the substrate in tape-form, which can be produced with the rapid solidification technique (by means of planar flow casting), can be obtained with a high productivity and with a particularly fine thickness. Furthermore, after the thermal treatment according to the present invention, it has high mechanical resistance properties and a limited reactivity under oxidative conditions, at the typical temperatures ranging from 800xc2x0 C. to 900xc2x0 C., at which the substrate must be maintained in the growth process of superconductor oxides.
In particular, an object of the present invention relates to the use of the end-articles in tape, sheet or fiber form as a reinforcing element in composite materials with an organic, metallic or pyroceram matrix, having an assembly temperature lower than or equal to 900xc2x0 C.
A detailed and illustrative description is provided below for a better understanding of the characteristics and advantages of the product and process according to the present invention.
The metal alloy consisting of nickel, chromium and metalloid elements, i.e. the quaternary alloy according to the present invention, comprises the following composition:
Nixe2x80x94Cr: (a total of 65 atom % to 84.5 atom %) with an atomic ratio between the metals Ni/Cr ranging from 1.5 to 4.5;
Bxe2x80x94Si: (a total of 15.2 atom % to 35 atom %) with Sixe2x89xa77.6 atom % and Bxe2x89xa7Si.
The metal alloy consists of nickel, chromium and metalloid elements, i.e. the quaternary alloy according to the present invention, can therefore also be represented by the following general formula:
NixCryBwSiz
wherein
x+y+w+z=100; x+y=65-84.8;
x/y=1.5-4.5; w/z greater than 1; 7.6 less than w less than 27.4.
The composition ranges are those indicated above and specifically:
Ni: 39.0-69.4 atom %;
Cr: 11.8-33.9 atom %;
B: 7.6-27.4 atom %;
Si: 7.6-17.5 atom %.
Accidental uneliminable impurities can be tolerated but less than 0.1% by weight.
The alloy of the composition according to the present invention can be obtained by means of the usual melting methods of the constituent elements or their partial alloys, and subsequent cooling, in an inert atmosphere, i.e. without gases such as oxygen or nitrogen which are reactive with respect to metals, or under vacuum.
End-articles can subsequently be prepared starting from this alloy, in the desired form, tape, film, sheet, fiber, having a thickness of less than 50 xcexcm and with a high productivity, in practically amorphous phase, preferably using the forming technology based on rapid solidification (for example melt spinning and planar flow casting).
These products must then be treated according to the particular thermal process, object of the present invention, to overcome their high fragility which occurs when the crystallization of the amorphous phase takes place at temperatures slightly higher than the glass transition temperature.
They are then subjected to thermal treatment which, in a particular application, can be carried out in an atmosphere of an inert gas or under vacuum at a temperature preferably ranging from 750xc2x0 C. to 880xc2x0 C., for a time varying from about 30 minutes (at the upper limit of the temperature range) to about 15 hours (at the lower limit of the temperature range).
If the treatment is carried out under vacuum, it preferably takes place at a pressure lower than 10xe2x88x924 mbars. If it is carried out in an inert gas, for example He or Ar, it can be effected at any pressure.
Following this treatment, the material is completely crystalline and is composed of a metallic-type matrix, prevalently made up of a solid solution, crystallized in the face-centered cubic system (FCC), and precipitates of microcrystalline nickel and chromium borides (Ni3B and CrB), as can be observed from X-ray diffraction analysis of the product. In the X-ray diffraction diagram the reflexes corresponding to the silicide phases are not clear and this leads to the conclusion that the specific characteristic of the metallic matrix having an FCC structure is that it is composed of an essentially ternary alloy based on Ni, Cr and Si.
The end-article thus obtained has high values relating to hardness, tensile modulus, yield point and ultimate tensile strength, together with a good ductility, even after being subjected to subsequent and repeated thermal treatment.
Vickers hardness values typical of the article obtained are 450 HV, typical ultimate tensile strength values are 1100 MPa, typical tensile modulus values are 170 GPa and the ductility is such that there is no breakage even when the bend radius is in the order of the tape thickness.
Furthermore, the end-article thus obtained is exceptionally resistant to oxidation up to high temperatures, in fact, if treated in air at 850xc2x0 C. for 1 hour its weight percentage increases by 0.14%, whereas a tape having an analogous thickness of a conventional alloy with a low oxidability, Nichrome 80/20, treated under the same condition, undergoes a weight percentage increase of 0.27%.
The main advantage of the article according to the present invention lies, as previously observed, in the combination of a low oxidability with high mechanical properties even at temperatures of about 600xc2x0 C.
The following examples provide a better illustration of the present invention.